1. Field of the Invention
The present invention relates to an echo canceller, andmore particularly to an echo canceller for use in, for example, a telephone terminal unit such as an audio teleconference terminal and a hands-free telephone set.
2. Description of the Background Art
Conventionally, a type of echo canceller is disclosed in Japanese Patent Laid-Open Publication No. 251079/1996. The Japanese Publication discloses a solution to an echo canceller mainly for use in a telephone switching system. More in detail, in the solution disclosed, when a hybrid circuit having an echo source impulse response characteristic is remotely located, the echo canceller estimates a delay up to the hybrid circuit, i.e. an initial delay on the echo path, from a signal output specific for measurement and a response signal thereto to assign a pure delay instead of filter coefficients of its adaptive filter and assign a predetermined length of taps residually held subsequent to the pure delay, thereby virtually shortening the tap length of the adaptive filter.
That solution relies upon the known fact that the longer tap length of an adaptive filter the longer period of its convergence time. That solution utilizes the fact that ones, assigned to the initial delay part of an echo path, of the taps of an adaptive filter do not substantially contribute to removing an echo.
However, if the solution disclosed in the Japanese '079 Publication is directly applied to an acoustic echo canceller, the acoustic echo would behave totally differently in echo path from the network echo so that the positional assignment of tap coefficients of an adaptive filter would still remain insufficient or excessive. This problem of the acoustic echo path is attributable to the fact that not only an initial delay but also an echo response after the initial delay, i.e. a scattering time response, would be extensively different from one to another. Therefore, the echo responses are different in length of trailing part.
From a different viewpoint, this problem is a natural and unavoidable phenomenon. The echo path of a network echo involves a response of a hybrid electric circuit that may be uniformly manufactured to a certain extent such as to satisfy the requirements of standardized telephone lines. In contrast, the acoustic echo path is a sound path on which part of a sound emitted from a loudspeaker is transmitted to be captured by a microphone. In other words, the acoustic echo is a response of sound reflection in a room including reflection caused with the presence of a near-end talker. It is therefore only natural such a response varies widely.
However, the solution disclosed in the Japanese '079 Publication has to have filters prepared in advance which have the respective filter lengths corresponding to the maximum length of echo paths possibly expected in practice in order to overcome the problem stated above. In a case where the initial delay and scatter response of an actual acoustic echo path are not very large, all of the residual long taps are assigned immediately thereafter. As a result, unnecessary filter coefficient taps would not be reduced. Thus, the period of convergence time is still long.
A further unfavorable problem is that, when an adaptive filter drives more taps than actually needed, such unnecessary taps may cause an error in echo estimation. Rather, the solution disclosed in the Japanese '079 Publication may often deteriorate the performance in removing the echo.
Moreover, even for the parts and calculations of an echo canceller causing such an error, filtering and coefficient updating are performed in exactly the same way as the parts and calculations originally necessary. This causes a digital signal processor (DSP) to perform unnecessary calculation and processing and consume excessive electric power therefor.
In order to overcome those underlying difficulties, U.S. Pat. No. 5,796,725 to Muraoka and Japanese Patent Laid-Open Publication No. 55687/1997 present solutions for controlling not only the initial delay but also shortening the tap length of an adaptive filter assigned to the scatter response part of an echo path.
In Muraoka, the degree of convergence of an adaptive filter is determined by the magnitude of a cancellation error, and, if the cancellation error is determined small, then a filter coefficient is reset which is smaller in value among the taps of the adaptive filter. Muraoka thus discloses a solution by applying no reset coefficient part to the adaptive filter, thereby reducing the number of taps.
The Japanese '687 Publication discloses an adaptive filter (ADF) having its tap coefficients grouped into several blocks, in which an initial delay is estimated in the initial operation, and thereafter it is determined on a block-by-block basis, while enabling the convergence to progress, whether or not the power of the tap coefficients exceeds a predetermined threshold value. A part exceeding the threshold value is used as a basic region for updating the ADF coefficient. Thereafter, while monitoring a residual signal increasing and decreasing, basically it is determined whether or not the deletion of the trailing block renders an echo return loss enhancement (ERLE) deteriorated and, if deteriorated, then the deleted block is restored. That procedure is repeated, thereby generally optimizing the taps
However, even these solutions often cause disadvantages. When applying the solution of Muraoka, a coefficient smaller in value than a predetermined threshold value is simply deleted. As a result, the taps are reduced in number, indeed. However, it may sometimes occur that the taps may be deleted so as to be thinned out, or even a correct tap coefficient incidentally small in value but appropriate for an input component may be removed, thereby deteriorating the performance. Moreover, when a residual is small, a part having a small coefficient is automatically deleted. However, in the case of a signal such as a voice signal fluctuable in reference input level, it is a natural phenomenon that a small residual signal simply involves the reference input level and an echo responsive thereto being small. Application of the solution of Muraoka causes such a natural phenomenon to be affected by a coefficient deletion, so that even a necessary coefficient is rendered deleted. Therefore, this solution involves such a problem that a sufficient echo cancellation may gradually become failed.
When applying the solution taught by the Japanese '687 Publication, in order to design the length of the blocks of ADF tap coefficients most appropriate for the size of a room or the like, an extensive proficiency is required, thus rendering it difficult to flexibly determine/design the bock length appropriate for an object to be applied to. Furthermore, a repetitive trial of increasing and decreasing the number of taps on a block-by-block basis results in the performance of echo removal being repeatedly deteriorated so as to cause the speech quality to be deteriorated, which is also problematic.
Against these difficulties, Japanese Patent Laid-Open Publication No. 2006-157498 discloses an excellent echo cancel solution in which the initial delay is calculated from the maximum value of coefficients of an adaptive filter to assign a delay in an initial delay part so as to save the tap length of a part corresponding to the initial delay of the adaptive filter, and further the adaptive filter has its taps grouped into blocks of different lengths, on each of which the sum-of-products value of the normalized power of the coefficients is calculated. It is determined in each block whether or not the normalized power becomes smaller as the more backward taps following the tap corresponding to the delay to thereby determine the adaptive filter converging. A predetermined threshold value is provided, and if the ratio in power of the rear blocks to the front blocks is smaller than the threshold value, the blocks are reduced whereas if the ratio is larger than the threshold value the blocks are increased, thus decreasing or increasing the tap coefficients on the block-by-block basis, with the result that the tap length of a scattering time section is also saved.
However, the solution of the Japanese '498 Publication also has a problem as described below. For example, when noise at the near-end talker is large so that a difference in power between the echo and the noise is small, a difference in power between the tap coefficients is also not significantly large. That may cause the blocks to be gradually increased toward the maximum length.
When the echo cannot be removed due to a noise in this way, it is preferable to reduce the tap length as short as possible to thereby save the resource of calculation and power consumption for the calculation.
However, when a difference in power between the noise and the echo is large and the boundary of the blocks coincides with the substantial part of the echo path, one block is reduced to cause an abrupt lack of the taps, and in turn the tap length is increased to update the coefficients in order to compensate for the lacking taps. The operation will be repeated. Thus, similar to the Japanese '687 Publication, a repetitive deterioration in performance of echo reduction may occur, which is problematic.